1. Field of the Invention
The invention pertains to a constant-voltage supply system having at least two constant-voltage sources, and more particularly, to such a system in which output voltage is supplied alternatively from one or another constant-voltage source.
2. Prior Art
A constant-voltage supply system in which output voltage is supplied alternatively from one or another constant-voltage source via a switched connection, is known from U.S. Pat. No. 4,492,875. In the event of failure of one voltage source, the system will automatically supply the voltage output from another source, and in this manner assures an uninterrupted supply of voltage. Such a system may be used to supply voltage to expensive high quality electronic systems such as computer systems or security equipment.
In the prior art, the state of the switched connection is controlled by control signals which are generated as a function of the operating state of the particular voltage source. For example, if the voltage of a first voltage source starts to drop below a given value, a control signal is generated which causes one switch to cut off the feeding of the voltage output from that voltage source. Furthermore, a second control signal is generated which drives another switch into the conducting state to establish a conducting path from a second voltage source to the voltage output. However, the first switch is completely turned off before the other switch is driven to the conducting state. In order to guarantee an uninterrupted voltage supply to the systems connected to the alternatively fed voltage output, a capacitor is provided which delivers the energy required to feed the voltage output during the switching interval. The size of the capacitor is determined by the maximum load connected to the voltage output, and can assume very high values. Such capacitors may be physically large and add to the expense and space requirements of the total system.
The switching operation does not occur abruptly, but over an extended switching time, since too steep a voltage rise at the voltage source can result in a current surge which overloads the voltage source. As a result of the extended switching time, a soft switching on or switching off of the voltage sources takes place. Switching time is to be understood as the transition time from the cutoff to the conducting state, and vice versa, of the switch.
It is the object of the invention to provide a voltage supply system which generates a largely uniform voltage level upon switch-over, and at the same time is independent of load.